Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. A number of metrology based techniques including scatterometry and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition and other parameters of nanoscale structures.
Traditionally, scatterometry critical dimension measurements are performed on targets consisting of thin films and/or repeated periodic structures. During device fabrication, these films and periodic structures typically represent the actual device geometry and material structure or an intermediate design. As devices (e.g., logic and memory devices) move toward smaller nanometer-scale dimensions, characterization becomes more difficult. Devices incorporating complex three-dimensional geometry and materials with diverse physical properties contribute to characterization difficulty. For example, modern memory structures are often high-aspect ratio, three-dimensional structures that make it difficult for optical radiation to penetrate to the bottom layers. Optical metrology tools utilizing infrared to visible light can penetrate many layers of translucent materials, but longer wavelengths that provide good depth of penetration do not provide sufficient sensitivity to small anomalies. In addition, the increasing number of parameters required to characterize complex structures (e.g., FinFETs), leads to increasing parameter correlation. As a result, the parameters characterizing the target often cannot be reliably decoupled with available measurements.
In one example, longer wavelengths (e.g. near infrared) have been employed in an attempt to overcome penetration issues for 3D FLASH devices that utilize polysilicon as one of the alternating materials in the stack. However, the mirror like structure of 3D FLASH intrinsically causes decreasing light intensity as the illumination propagates deeper into the film stack. This causes sensitivity loss and correlation issues at depth. In this scenario, SCD is only able to successfully extract a reduced set of metrology dimensions with high sensitivity and low correlation.
In another example, opaque, high-k materials are increasingly employed in modern semiconductor structures. Optical radiation is often unable to penetrate layers constructed of these materials. As a result, measurements with thin-film scatterometry tools such as ellipsometers or reflectometers are becoming increasingly challenging.
In response to these challenges, more complex optical metrology tools have been developed. For example, tools with multiple angles of illumination, shorter illumination wavelengths, broader ranges of illumination wavelengths, and more complete information acquisition from reflected signals (e.g., measuring multiple Mueller matrix elements in addition to the more conventional reflectivity or ellipsometric signals) have been developed. However, these approaches have not reliably overcome fundamental challenges associated with measurement of many advanced targets (e.g., complex 3D structures, structures smaller than 10 nm, structures employing opaque materials) and measurement applications (e.g., line edge roughness and line width roughness measurements).
Atomic force microscopes (AFM) and scanning-tunneling microscopes (STM) are able to achieve atomic resolution, but they can only probe the surface of the specimen. In addition, AFM and STM microscopes require long scanning times. Scanning electron microscopes (SEM) achieve intermediate resolution levels, but are unable to penetrate structures to sufficient depth. Thus, high-aspect ratio holes are not characterized well. In addition, the required charging of the specimen has an adverse effect on imaging performance. X-ray reflectometers also suffer from penetration issues that limit their effectiveness when measuring high aspect ratio structures.
To overcome penetration depth issues, traditional imaging techniques such as TEM, SEM etc., are employed with destructive sample preparation techniques such as focused ion beam (FIB) machining, ion milling, blanket or selective etching, etc. For example, transmission electron microscopes (TEM) achieve high resolution levels and are able to probe arbitrary depths, but TEM requires destructive sectioning of the specimen. Several iterations of material removal and measurement generally provide the information required to measure the critical metrology parameters throughout a three dimensional structure. But, these techniques require sample destruction and lengthy process times. The complexity and time to complete these types of measurements introduces large inaccuracies due to drift of etching and metrology steps. In addition, these techniques require numerous iterations which introduce registration errors.
Transmission, Small-Angle X-Ray Scatterometry (T-SAXS) systems have shown promise to address challenging measurement applications. Current T-SAXS tools employ a beam forming slits to form the illumination beam incident on the specimen under measurement. A beam divergence shaping slit is located in the beam path before or after the focusing optics to define the divergence angle of the beam. A beam shaping slit is located in the beam path after the beam divergence shaping slit to define the size of the beam spot incident on the wafer.
Unfortunately, available x-ray sources have a finite dimension in directions orthogonal to the direction of beam propagation. Due to finite source size, the beam spot incident on the specimen will be defined by the size of the beam shaping slit and the angular dimension of the source from the optics (e.g., focusing optics, collimating optics, etc.). For example, the size of the image of an x-ray source in a focal plane of an optical system is defined by its actual size and the magnification of the optics. The magnification of the optics is the ratio of the distance from the focusing optic to the image and the distance from the focusing optic to the source. In addition, slope and figure errors of the focusing optics will further increase the beam spot size. Current systems do not meet the requirements for measurements of metrology targets located in scribe lines where a beam spot size of 50 micrometers, or less, is required.
To address this problem, it is possible to reduce the size of the beam shaping slit. However, this results in a dramatic reduction of photon flux, which makes renders the measurements ineffective. Furthermore, reducing the size of the beam shaping slit does not completely solve the problem because beam divergence always contributes to beam spread at the point of incidence with the specimen under measurement. For example, in typical T-SAXS systems the beam shaping slit is more than 250 millimeters from the surface of specimen under measurement. For typical beam divergence present in these systems, a beam spot size of 30-40 micrometers is expected even if the size of the beam shaping slit were infinitesimally small. Of course, this arrangement is impractical because an infinitesimally small amount of illumination would be projected onto the specimen if a measurement system were configured in such a manner.
The impact of beam divergence on beam spot size can be reduced by locating the beam shaping slit closer to the specimen. However, in current practice, this has not been achieved for T-SAXS systems. An effective T-SAXS metrology system performs measurements of a specimen oriented at different angles of incidence with respect to the incoming beam. In other words, the specimen is tilted with respect to the incoming beam such that a surface normal of the specimen is oriented from the beam axis of the illumination beam by as much as 30 degrees, or more. Under these conditions, traditional beam shaping slits mechanically interfere with the specimen if they are not spaced apart from the specimen by a significant distance. In current systems implemented by KLA-Tencor Corporation, the distance between the beam shaping slit and the specimen under measurement is 260 millimeters.
U.S. Pat. No. 7,406,153 describes a grazing incidence tool which employs knife edge beam blocks in close proximity to the specimen under measurement. However, the disclosed beam blocks are only functional in the context of a grazing incidence tool, not a T-SAXS tool where normal illumination, or illumination at angles up to 50 degrees from normal are required.
To further improve device performance, the semiconductor industry continues to focus on vertical integration, rather than lateral scaling. Thus, accurate measurement of complex, fully three dimensional structures is crucial to ensure viability and continued scaling improvements. Future metrology applications present challenges for metrology due to increasingly small resolution requirements, multi-parameter correlation, increasingly complex geometric structures including high aspect ratio structures, and increasing use of opaque materials. Thus, methods and systems for improved T-SAXS measurements are desired.